evolution of a strategy for the synthesis of structurally complex batzelladine alkaloids....

Evolution of a Strategy for the Synthesis of StructurallyComplex Batzelladine Alkaloids. Enantioselective Total

Synthesis of the Proposed Structure of Batzelladine F andStructural Revision

Frederick Cohen† and Larry E. Overman*

Contribution from the Department of Chemistry, 516 Rowland Hall, UniVersity of California,IrVine, California 92697-2025

Received October 31, 2005; E-mail: [email protected]

Abstract: Stereoselective synthesis of octahydro-5,6,6a-triazaacenaphthalenes 29 and 34 having the anti-relationship of the angular hydrogens flanking the pyrrolidine nitrogen confirmed suspicions that the relativeconfiguration of the left-hand tricyclic guanidine fragment of batzelladine F should be revised to have thesyn relationship of these hydrogens. Several strategies were examined for coupling tricyclic guanidinefragments to prepare potential structures for batzelladine F. Eventually, a convergent synthesis strategywas devised, whose central step was a fragment-coupling tethered-Biginelli reaction (Scheme 17). Usingthis approach we synthesized four potential structures of batzelladine F, 35-38. None of these compounds,nor their enantiomers, were identical to natural batzelladine F. Reinvestigation of mass spectra of naturalbatzelladine F, and fragments 88 and 89 obtained upon saponification of batzelladine F, demonstratedthat the originally proposed connectivity of this alkaloid was also incorrect. The revised connectivity, 90, ofnatural batzelladine F depicted in Scheme 21 is proposed.

Introduction

In 1997, Patil and co-workers reported the isolation ofbatzelladines F-I from a red Jamaican sponge incorrectlyidentified at the time asBatzella sp.;1,2 the structures1-4assigned originally to these alkaloids are shown in Figure 1.Each of these natural products contain two tricyclic guanidinesand were reported to induce dissociation of the tyrosine kinasep56lck from CD4. It was postulated that disruption of thisinteraction could be used to treat autoimmune diseases. As partof our larger program in the synthesis of guanidinium alkaloids,3

we were attracted to batzelladine F for two reasons. First, wewere interested in preparing compounds that inhibit specificprotein-protein interactions involving large surface areas, and,second, we sought to define totally the relative and absoluteconfiguration of batzelladine F, much of which was unclear atthe outset of this work.4

The original structural proposal1 for batzelladine F was basedon the following analysis. The relative configuration of the right-

hand tricyclic portion (C18 through C28) of batzelladine F wasassigned by comparison of its13C NMR spectra to that of thetricyclic guanidine alkaloid batzelladine D, the structure of

† Department of Medicinal Chemistry, Genentech, Inc., 1 DNA Way,South San Francisco, CA 94080.(1) Patil, A. D.; Freyer, A. J.; Taylor, P. B.; Carte´, B.; Zuber, G.; Johnson, R.

K.; Faulkner, D. J.J. Org. Chem. 1997, 62, 1814-1819.(2) The identity of the sponge has been revised: Braekman, J. C.; Daloze, D.;

Tavares, R.; Hajdu, E.; Van Soest, R. W. M.J Nat. Prod. 2000, 63, 193-196.

(3) For a recent review, see: Aron, Z. D.; Overman, L. E.Chem. Commun.2004, 253-265.

(4) (a) For reviews summarizing the isolation, structure and synthesis ofbatzelladine alkaloids, see: Berlinck, R. G. S.; Kossuga, M. H.J. Nat.Prod. 2005, 22, 516-550 and earlier reviews in this series. (b) For therecent isolation of a new batzelladine alkaloid, see: Gallimore, W. A.;Kelly, M.; Scheuer, P. J.J. Nat. Prod.2005, 68, 1420-1423. (c) For arecent synthetic study, see: Arnold, M. A.; Duron, S. G.; Gin, D. Y.J.Am. Chem. Soc.2005, 127, 6924-6925.

Figure 1. Proposed Structures of Batzelladines F-I.

Published on Web 02/02/2006

2594 9 J. AM. CHEM. SOC. 2006 , 128, 2594-2603 10.1021/ja0574320 CCC: $33.50 © 2006 American Chemical Society

which had been defined by total synthesis.5,6 The configurationof the left-hand tricyclic portion (C1 through C9) was assignedon the basis of NOE data.7 The length of the nonyl chain atC25, and the length of the chain connecting the tricyclicguanidines were assigned by MS fragmentation patterns, butthis analysis was not discussed, nor were the data presented.There was no information that related the relative configurationsof the two tricyclic guanidine fragments, specified the config-uration at C16, or defined the absolute configuration.

In 1999, Snider and Murphy independently published syn-theses of tricyclic guanidines related to the left-hand portion ofbatzelladine F that have a syn relationship of the methinehydrogens flanking the pyrrolidine nitrogen.8 The13C NMR dataof these analogues supported a revision of the configuration ofthe natural product at C4 and C7 from anti to syn. However,rigorous verification of this proposal would require the synthesisof a related anti-fused tricyclic guanidine and the demonstrationthat its spectra were distinct from those of the natural product.

In this contribution, we first describe our synthesis of 2a,-8a-anti tricyclic guanidines corresponding to the originally pro-posed relative configuration of these units of batzelladine F andproof that the original configurational assignment of the left-hand guanidine fragment of this alkaloid was incorrect (SectionI). We then describe our preparation of analogous 2a,8a-syntricyclic guanidines (Section II) and the evolution of our strategyfor the total synthesis of linked tricyclic guanidines correspond-ing to the proposed constitution and revised relative configu-ration of batzelladine F (Sections III-IV). We then demonstratethat the originally reportedconnectiVity of the natural product

was incorrect, leading to a new proposal for the structure ofbatzelladine F (Section V). In the following contribution, wedetail our synthesis of this new structure, and studies that defineall aspects of the structure of batzelladine F.9

Results and Discussion

I. Synthesis of 2a,8a-Anti-decahydrotriazaacenaphthalenesCorresponding to the Originally Proposed Structure ofBatzelladine F. Believing that both tricyclic guanidine frag-ments of batzelladine F possessed the same anti relationshipbetween the angular hydrogens at carbons 4 and 7, and carbons20 and 23 (batzelladine numbering), we conceived of a divergentstrategy that would allow us to construct each half of themolecule from a common intermediate. We chose the ester bondas a logical first disconnection, thus, generating alcohol5 andacid6 as synthetic objectives. Each of these intermediates wasconceived as arising from an intermediate such as7a or 7b,which differ in the side chain at C9. Employing a disconnectiondeveloped during our total synthesis of batzelladine D,6

intermediates7aand7b were simplified to a common precursor8 bearing a masked aldehyde that would allow for attachmentof different side chains by olefination reactions. Anti aminoalcohol8 was seen as arising fromâ-hydroxy ester9.

The preparation of common intermediate8 began withWeinreb amide10,10 to which Grignard reagent1111 was addedto afford â-hydroxy ketone12 in 76% yield (Scheme 2). Thisintermediate was subjected to Evans’ variant of the Tishchenkoreduction to set the 1,3-stereorelationship and differentiate thehydroxyl groups of the resulting 1,3-diol.12 In this way,monoprotected diol13 was obtained in 92% yield from ketoneprecursor12 as a single stereoisomer. Nitrogen was installed

(5) Snider, B. B.; Chen, J.; Patil, A. D.; Freyer, A. J.Tetrahedron Lett. 1996,37, 6977-6980.

(6) Cohen, F.; Overman, L. E.; Sakata, S. K. L.Org. Lett. 1999, 1, 2169-2172.

(7) These data were not presented in the paper or the Supporting Information.(8) (a) Snider, B. B.; Busuyek, M. V.J. Nat. Prod. 1999, 62, 1707-1711. (b)

Black, G. P.; Murphy, P. J.; Thornhill, A. J.; Walshe, N. D. A.; Zanetti, C.Tetrahedron1999, 55, 6547-6554.

(9) (a) Our synthesis of the natural product has been outlined: Cohen, F.;Overman, L. E.J. Am. Chem. Soc. 2001, 123, 10782-10783. (b) Cohen,F.; Overman, L. E.J. Am. Chem. Soc.2006, 128, 2604-2608.

(10) Coffey, D. S.; McDonald, A. I.; Overman, L. E.; Stappenbeck, F.J. Am.Chem. Soc. 1999, 121, 6944-6945.

(11) Lee, T. V.; Porter, J. R.Org. Synth. 1995, 72, 189-195.(12) Evans, D. A.; Hoveyda, A. H.J. Am. Chem. Soc. 1990, 112, 6447-6449.

Scheme 1 Scheme 2

Enantioselective Total Synthesis of Batzelladine F A R T I C L E S

J. AM. CHEM. SOC. 9 VOL. 128, NO. 8, 2006 2595

in the form of an azide, by Mitsunobu inversion of alcohol13with hydrazoic acid to provide azide14 in 95% yield.13,14 The1,3-anti configuration was then reestablished by methanolysisof propionate14, followed by Mitsunobu inversion of theproduct alcohol withp-nitrobenzoic acid15 to deliver aryl ester15 in 78% yield for the two steps. Ozonolysis of unsaturatedazido ester15 in a mixture of MeOH, CH2Cl2 and NaHCO3

provided aldehyde16, setting the stage for attachment of thedifferent tricyclic guanidine side chains.16

After aldehyde16 failed to react cleanly with the ylide derivedfrom a simple hexyl phosphonium salt, we turned to theKocienski modification of the Julia olefination.17 This procedurerequired the synthesis of an appropriate sulfone partner (Scheme3). Thus, diol17 was monoprotected by reductive opening ofthe corresponding benzylidine acetal to provide alcohol18 in94% yield and 96% ee.18,19Activation of the alcohol as a tosylateallowed displacement with mercaptotetrazole19. Oxidation ofthe resulting sulfide withm-chloroperbenzoic acid (m-CPBA)in a mixture of CH2Cl2 and pH 7 buffer afforded sulfone20 in72% yield for the three steps.

Aldehyde 16 was then elaborated to amino alcohol22 assummarized in Scheme 4. Union of sulfone20 with aldehyde16 was accomplished by addition of a THF solution of thealdehyde to a small excess of the lithium reagent derived fromsulfone20 at -50 °C. Alkene21 was obtained in∼75% yieldas a 3:2 mixture of stereoisomers. This product was contami-nated with residual sulfone20, which could be removed morereadily after hydrolysis of the ester. The azide and double bondwere then reduced concurrently with hydrogen over palladiumpoisoned with ethylenediamine.20 As expected, the benzyl etherwas stable to this reduction. In this fashion, amino alcohol22was obtained in 73% overall yield from aldehyde16.

Employing chemistry developed during our earlier synthesisof the guanidine core of batzelladine D,6 the conversion of aminoalcohol22 to the corresponding tricyclic guanidine26 took placesmoothly (Scheme 5). Guanylation of amine22 with reagent23,22 followed by hydrolysis of the acetal provided guanidine hemi aminal 24. This labile intermediate was immediately

condensed with allyl acetoacetate to provide Biginelli product25 in 45% yield as an 8:1 mixture of stereoisomers.23 Afterseparation of the minor syn stereoisomer by chromatography,mesylation of the anti product and ring closure provided tricyclicguanidine26 in 78% yield.

With allyl ester 26 in hand, we sought to investigate itsdecarboxylation. Such transformations of simple vinylogous

(13) Mitsunobu, O.Synthesis1981, 1-32.(14) Hassner, A.; Fibiger, R.; Andisik, D. J.J. Org. Chem. 1984, 49, 4237-

4244.(15) Martin, S. F.; Dodge, J. A.Tetrahedron Lett. 1991, 32, 3017-3020.(16) Without this basic additive, the bis-dimethyl acetal was isolated in

quantitative yield. See: Woodward, R. B. et al.J. Am. Chem. Soc.1981,103, 3210-3213.

(17) Blakemore, P. R.; Cole, W. J.; Kocienski, P. J.; Morley, A.Synlett1998,26-28.

(18) Seebach, D.; Zuger, M.HelV. Chem. Acta. 1982, 65, 495-503.(19) Volkov, A. A.; Zlotski, S. S.; Kravets, E. K.; Spirknin, L. V.Chem.

Heterocycl. Compd. (Engl. Trans.)1985, 21, 861-863.(20) Sjiki, H.; Hattori, K.; Hirota, K.J. Org. Chem. 1998, 63, 7990-7992.(21) Franklin, A. S.; Ly, S. K.; Mackin, G. H.; Overman, L. E.; Shaka, A. J.J.

Org. Chem. 1999, 64, 1512-1519.

(22) Bernatowicz, M. Z.; Wu, Y.; Matsueda, G. R.J. Org. Chem. 1992, 57,2497-2502.

(23) Stereoselection in the tethered Biginelli condensation has been thoroughlyinvestigated: McDonald, A. I.; Overman, L. E.J. Org. Chem. 1999, 64,1520-1528.

Scheme 3 Scheme 4

Scheme 5

A R T I C L E S Cohen and Overman

2596 J. AM. CHEM. SOC. 9 VOL. 128, NO. 8, 2006

carbamates were precedented, typically involving heating thefree acid to>100°C in the presence of an acid or catalyst suchas cyanide.24 We were delighted to find that reaction of ester26with catalytic (PPh3)4Pd and pyrrolidine in a mixture of THFand MeOH resulted within an hour in cleavage of the allyl esterand concomitant decarboxylation.25 The intermediate enamineabsorbed one equivalent of MeOH to yield hemi aminal27.Without isolation, this intermediate was reduced with NaBH3-CN in AcOH to afford saturated tricyclic guanidine28 in 67%yield. Stereoselection in this reduction results from axial deliveryof hydride to theN-amidinyl iminium ion derived from27.26

Finally, the benzyl ether of28 was removed by hydrogenolysisto provide alcohol29.

The facility with which guanidine ester26 decarboxylatesafter deallylation can be explained by postulating an intermediatesuch as31, which would arise from tautomerization of30(Scheme 7). The breaking bond in guanidnium carboxylate31would be aligned well with the iminiumπ-system.

We were now in a position to compare the NMR spectra ofour tricyclic guanidine products, batzelladine F, and Murphy’ssyn-fused model compound33 (Table 1).27 The 13C NMRspectra of the two anti-fused tricyclic guanidines29 and3428

match each other almost perfectly yet differ somewhat fromthose of the natural product. In contrast, the13C NMR data for33match those of the natural product closely. It was clear fromthis comparison that the proposed anti relationship of the angularhydrogens of the left-hand guanidine fragment of batzelladineF was incorrect: the actual configurational relationship is indeedsyn.

II. Synthesis of 2a,8a-Syn-decahydrotriazaacenaphtha-lenes. Although the relative configuration of the left-hand

tricyclic portion of batzelladine F was now firmly establishedas syn, no information was available regarding the relativeconfiguration of the two guanidine fragments, the relativeconfiguration of the methyl substituent of the linker, nor theabsolute configuration. Thus, there were eight compounds35-38 (four pairs of enantiomers) that fit all of the available datafor batzelladine F (Figure 2).

We turned next to develop a convenient synthesis of the left-hand tricyclic guanidine portions of35-38. The approach wechose was founded on the chemistry we had developed earlierto synthesize the tricyclic moiety of batzelladine B,21 in thiscase a tethered Biginelli reaction would be employed to combinefragments41 and42 (Scheme 8). Both of these intermediateswe saw being available from the same starting material, methyl(R)-3-hydroxybutarate (44).

The synthesis ofâ-keto ester52 began with (R)-iodide 45,which was prepared in three routine steps from ester44 (Scheme9).29 Alkylation of this iodide with the lithium enolate oftert-butyl acetate proceeded well on a small scale.30 On larger scale,the desired product46was contaminated with varying amountsof the dialkylation product47, which was difficult to remove.Reduction of this mixture with LiAlH4 provided a mixture of

(24) Reuman, M.; Eissenstat, M. A.; Weaver, J. D., IIITetrahedron Lett. 1994,35, 8303-8306, and references therein.

(25) A model compound similar to26 bearing a methyl ester was completelyresistant to both basic and acidic hydrolysis.

(26) Snider, B. B.; Chen, J.; Patil, A. D.; Freyer, A. J.Tetrahedron Lett. 1996,37, 6977-6980.

(27) Black, G. P.; Murphy, P. J.; Walshe, N. D. A.Tetrahedron1998, 54, 9481-9488.

(28) Synthesized from unsaturated azido benzoate15 in a fashion analogous to29; see Supporting Information.

(29) Overman, L. E.; Rabinowitz, M. H.J. Org. Chem. 1993, 58, 2335-3237.(30) Heathcock, C. H.; Pietter, S.; Ruggeri, R. B.; Ragan, J. A.; Kath, J. C.J.

Org. Chem. 1992, 57, 2554-2566.

Scheme 6 Scheme 7

Table 1. Comparison of 13C Chemical Shifts of Batzelladine F andTricyclic Model Compoundsa

carbonno. 29 34 1 33

1 21.7 21.7 20.7 20.72 48.9 48.6 47.2 47.34 56.5 56.5 57.5 57.57 56.4 56.3 57.4 57.59 53.1 52.8 51.6 51.6

a 13C NMR in CD3OD (500 MHz).



primary alcohols, from which the desired product48 could beisolated cleanly.31 Conversion of alcohol48 to iodide 49 setthe stage for elaboration to aâ-keto ester. Thus, addition of thedianion of methyl acetoacetate (50) to iodide 49 proceededcleanly, delivering51 in 68% yield.32 Transesterification of thisproduct with allyl alcohol gave allyl ester52 in nearlyquantitative yield.33

Guanidine hemi aminal41 was assembled by the sequencesummarized in Scheme 10. Hydroxybutyrate44 was converted

first to Weinreb amide53 in 80% yield using the Merckprocedure (Scheme 10).34 Addition of Grignard reagent11 tothis amide provided hydroxy ketone54, which was reduced withEt2BOMe and NaBH4 to provide syn-diol55 as a singlestereoisomer.35 Double Mitsunobu reaction of this intermediatewith hydrazoic acid, followed by hydrogenation of the diazideproduct over Pd‚C provided diamine43 in 80% yield. Conver-sion of this intermediate to Troc-protected guanidine57 wasaccomplished with reagent56 in 82% yield.36

To set the stage for the Biginelli condensation, the Troc groupand the dimethyl acetal were removed from guanidine57simultaneously by reaction with Zn dust in aqueous AcOH.

(31) Several lengthier synthesis of this compound are reported: (a) Nokami, J.;Taniguchi, T.; Ogawa, Y.Chem. Lett. 1995, 43-4. (b) Solladie, G.; Lohse,O. J. Org. Chem. 1993, 58, 4555-4563. (c) Ernst, B.; Wagner, B.HelV.Chim. Acta. 1989, 72, 165-171.

(32) Huckin, S. N.; Weiler, L.J. Am. Chem. Soc.1974, 96, 1082-1087.(33) Taber, D. F.; Amedio, J. C.; Patel, Y. K.J. Org. Chem. 1985, 50, 3618-

1619.

(34) Williams, J. M.; Jobson, R. B.; Yasuda, N.; Marchesini, G.; Dolling, U.-H.; Grabowski, E. J. J.Tetrahedron Lett. 1995, 36, 5461-5464.

(35) Chen, K. M.; Hardmann, G. E.; Prasad, K.; Repic, O.; Shapiro, M. J.Tetrahedron Lett. 1987, 28, 155-8.

(36) Atkins, P. R.; Glue, S. E. J.; Kay, I. T.J. Chem. Soc., Perkin Trans. 11973, 2644-2646.

Figure 2. Proposed revised constitution of batzelladine F (90) and theconfigurational uncertainties yet to be resolved. Plausible Structures forBatzelladine F as of 1999.

Scheme 8

Scheme 9

Scheme 10



However, removal of residual zinc salts from the resulting crudeguanidine hemi aminal41 proved to be problematic. Forexample, saturation of the reaction with H2S formed a thickprecipitate, presumably ZnS, which was difficult to filter.Moreover, the polar nature of guanidine salt41 caused it toadhere to these salts. At most, 40% of the desired product couldbe recovered from this reaction. Therefore, we prepared thebenzyl imino thiocarbamate58 (see Supporting Information fordetails), and condensed it with diamine43 to form the Cbz-protected guanidine59 in 82% yield. Hydrogenolysis of thisintermediate removed the Cbz group. Hydrolysis of the resultingguanidinium acetal proceeded cleanly, albeit slowly, at roomtemperature in aqueous acetic acid to provide bicyclic hemiaminal41 in nearly quantitative yield over the two steps.

With the two tethered Biginelli reaction partners in hand, theconstruction of 2a,8a-syn-decahydrotriazaacenaphthalene39proceeded uneventfully (Scheme 11). Guanidine41 condensedsmoothly with an excess ofâ-keto ester52, under previouslyoptimized conditions,21 to provide tricyclic guanidine60 in 81%yield as a 5:1 mixture of syn and anti stereoisomers. As in theanti series, the unnecessary ester appendage was removed bybrief treatment of Biginelli product60 with (Ph3P)4Pd andpyrrolidine, followed by reduction of the intermediate hemiaminal with NaBH4 in AcOH. Workup of this reaction with 1N HCl then removed the silyl group. After chromatography onSiO2, the product was treated with aqueous NaBF4 to provideguanidine alcohol39 having a BF4- counterion in 66% overallyield from ester 60. As expected, the diagnostic methineresonances in the13C NMR spectrum of syn-decahydrotriaz-aacenaphthalene39 matched almost exactly those signals ofbatzelladine F and Murphy’s model compounds (see SupportingInformation).

III. Synthesis of the Revised Proposed Structure ofBatzelladine F: R-Bromo Acid Strategy. The preparationof the right-hand 2a,8a-anti-decahydrotriazaacenaphthalene-carboxylate is summarized in Scheme 12. This synthesis beganwith hemi aminal61,6 which was condensed withtert-butylacetoacetate under conditions optimized for anti stereoselection.This reaction provided anti bicyclic guanidine62, which wasisolated as the acetate salt after chromatography on silica gel.Conversion of this intermediate to the corresponding mesylatewas complicated byN-sulfonylation of the guanidine. Eventu-

ally, we found that exchanging the acetate counterion ofguanidine62 for tetrafluoroborate allowed for clean mesylationof the alcohol upon reaction with MsCl and Et3N in CH2Cl2.This mesylate cyclized in hot CHCl3 in the presence of excessEt3N to provide tricyclic guanidine ester63 in 62% overall yield.As expected,6 hydrogenation of63 over Rh‚Al2O3 proceededwith little stereoselection. The resulting mixture of esters wasimmediately deprotected with HCO2H to provide the corre-sponding acids. These isomers were separated by reverse-phaseHPLC to provide stereoisomer64, having the same relativeconfiguration as batzelladine D and the right-hand guanidinefragment of batzelladine F, in 30% yield along with 48% ofstereoisomer65.

With the two tricyclic guanidine fragments available, theirunion was investigated. Using the more abundant isomer65 asa model, a variety of coupling reagents were evaluated, includingdiimides, DEAD-Ph3P, and activated esters.37 The only condi-tions that showed promise involved the use of 2-chloro-1-methylpyridinium iodide (Mukaiyama’s salt) and DMAP inMeCN.38 Coupling of acid65 and alcohol39 in this wayprovided ester66 in 48% yield after purification by HPLC(Scheme 13).

With a successful coupling procedure in hand, we turned toprepare the hexacyclic diguanidine35 depicted in Figure 2(Scheme 14). The desired coupling of guanidine alcohol39andguanidinium carboxylate64 did not take place at 50°C or 75°C.37 However, when this coupling reaction was conducted at100°C, ester68was formed in 57% yield. Unfortunately, uponcharacterization, it became apparent that68 had undergoneepimerization at C19. Particularly diagnostic was the1H NMRsignal of C19, which appeared as an apparent triplet atδ 2.38(t, J ) 10.3 Hz, CD3OD); whereas authentic batzelladine F

(37) These reactions were conducted on a small scale (<1 mg) and analyzedby HPLC-MS.

(38) Mukayama, T.; Usui, M.; Shimada, E.; Saigo, K.Chem. Lett. 1975, 1045-1048.

Scheme 11 Scheme 12



shows the corresponding resonance atδ 3.06 (dd,J ) 4.6, 3.3Hz, CD3OD). It was subsequently discovered that acid64 rapidlyepimerizes under these reaction conditions, and the resultingepimeric acid,67, couples to alcohol39 rapidly at 50°C. Theseresults lead us to believe that epimerization preceded coupling.The failure of 64 to couple under these conditions and itsepimerization under more forcing conditions is readily attributedto the hindered axial nature of the carboxylic acid substituent.

We turned to explore whether the ester could be epimerizedafter coupling. In our synthesis of batzelladine D, an attemptwas made to epimerize a related ester by kinetic protonation ofan enolate intermediate.6 Although yields and selectivities werelow in this earlier study, we felt that this strategy warrantedreinvestigation. The desired configuration of the diguanidineester would arise if the enolate of ester68 protonated from theless-hinderedâ face.

Before embarking on such an ambitious undertaking, wesought to explore the epimerization step in a model system.We anticipated that success would require that at least some ofthe NH hydrogens of the guanidine groups be masked. Thus,the tricyclic guanidine ester69 depicted in eq 1 was assembledby reduction ofR,â-unsaturated ester63 with NaBH3CN inAcOH, followed byN-benzylation by reaction with KHMDSand benzyl bromide. We were disappointed to find that reactionof this ester with a variety of bases (LDA, LiNEt2, KHMDS,BuLi), followed by quenching with a deuterium source resulted

in no deuterium incorporation (eq 1). In contrast, reaction ofguanidine ester69 with excesstert-butyllithium at -78 °C inTHF, followed by quenching the reaction with CD3OD resultedin 100% deuteration. However,1H NMR showed to our surprisethat deuterium had been introduced at the benzylic methylenegroup.39

Unable to generate an enolate by deprotonation, we soughtto access this intermediate in a slightly more circuitous manner.We anticipated that a bromine substituentR to the ester wouldallow the enolate required for epimerization to be generated byreduction. The synthesis of the bromo acid required to inves-tigate this strategy began with unsaturated ester63, which wastreated withN-bromosuccinimide (NBS) in MeOH to generatebromo ether 71 in quantitative yield (Scheme 15). This

intermediate was reduced immediately with NaBH3CN in AcOHto provideR-bromo ester72. This reduction was plagued bycompeting elimination of “BrOMe” to return unsaturated ester63, which under the reaction conditions was reduced to ester73.40 Cleavage of thetert-butyl group of ester72 with 98%HCO2H, and purification of the product by HPLC furnishedthe desired acid74 in 47% overall yield. Although notestablished rigorously, the configuration indicated for74wouldarise from diaxial addition of MeOBr to63 and axial reductionof the N-amidinyl iminium ion generated from71.

(39) Molecular mechanics modeling shows that the ester group of intermediatesof this type exists preferntially in a conformation in which the methinehydrogen and the carbonylπ-system are nearly orthogonal. This feature islikely responsible for the failure to generate the ester enolate of69.

(40) Other reductive conditions (NaBH(OAc)3, AcOH.; NaBH4, EtOH, 98%HCO2H, Et3SiH, TFA) either failed to reduce71 or led to decomposition.

Scheme 13

Scheme 14

Scheme 15



With guanidineR-bromo acid74 available, its coupling toguanidine alcohol39 was investigated (Scheme 16). UsingMukaiyama’s salt, coupling proceeded to a small extent (∼20%,HPLC-MS analysis) after 24 h at 50°C. Attempts to force thereaction by increasing the temperature to 85°C provided amixture of diguanidines75 and76 in which both eliminationof HBr and coupling had taken place. These double bondisomers could be separated by HPLC, and were readilydistinguished by1H NMR spectroscopy.41

The formation of76 finally suggested a potentially straight-forward route for accessing batzelladine F: couple the guanidinefragments prior to saturating the right-hand tricyclic guanidinefragment. We proceeded to test the last stage of this strategywith diguanidine76, which was now available (Scheme 17).Hydrogenation of76 over Rh‚Al2O3 provided stereoisomericdiguanidines35 and77, which could be separated by HPLC.Although compound35 showed 1H and 13C NMR spectraconsistent with those of natural batzelladine F, it was substan-tially different from the natural product by HPLC analysis.

Given that we had only a 25% chance of making the correctrelative stereoisomer at the outset, it was not surprising that35was not batzelladine F. At this point, we were faced with twoissues. First was the problem of efficiently linking the guanidinefragments and installing the desired configuration at C19.Second was the identity of the natural product. Although it wasnot immediately clear how the former could be solved, a latestage catalytic hydrogenation strategy should permit us to pre-pare the remaining three stereoisomers, allowing for elucidationof the relative and absolute configuration of the natural product.

IV. Fragment Coupling Biginelli Strategy for the Synthesisof the Revised Proposed Structure of Batelladine F andStereoisomers.Having decided to pursue a synthesis strategywith saturation of the C19-C27 double bond as the final step,we realized that there was a shorter route to such late-stageintermediates than theR-bromo acid coupling strategy. Thisapproach is outlined retrosynthetically in Scheme 17. As in oursynthesis of batzelladine D, we envisaged the right-hand tricyclicguanidine fragment of1 evolving from pentacyclic diguanidine78by ring closure followed by hydrogenation. This intermediatewould be the product of a highly convergent tethered Biginellicondensation betweenâ-keto ester79 and guanidine hemi-aminal61. Ester79 would arise from alcohol80.

To pursue the strategy outlined in Scheme 17, we began withguanidine alcohol81 whose tricyclic guanidine moiety isenantiomeric to the guanidine fragment of alcohol39. Alcohol81, which was prepared analogously to39,42 was acylated withmethyl acetoacetate in the presence of DMAP to provideâ-ketoester82 in quantitative yield (Scheme 18).33 This material wascombined with an excess of guanidine aminal61 and morpho-linium acetate in 2,2,2-trifluoroethanol and heated at 60°C for48 h. Pentacyclic diguanidine83 was obtained in 64% yieldafter separation from residual61 and minor amounts (e10%)of isomer84.

(41) The signals in the1H NMR spectrum of76 corresponding to the left-handtricyclic portion were nearly identical to that of68. The two most diagnosticpeaks were the vinyl methyl singlet atδ 2.26 and the allylic methine atδ4.25 (dd,J ) 11.7, 5.2 Hz). Both of these resonances were absent fromthe spectrum of75, and were replaced with an allylic methine atδ 4.49 (q,J ) 6.3 Hz) and a multiplet atδ 2.45-2.30 corresponding to the allylicmethylene. (42) Details are provided in the Supporting Information.

Scheme 16 Scheme 17



To complete the synthesis of the hexacyclic diguandine,we needed to close the final ring and saturate the C19-C27double bond (Scheme 19). To this end, the triflouroacetatecounterions of intermediate83 were exchanged for tetrafluo-roborate by washing a CHCl3 solution of 83 with aqueousNaBF4. This diguanidine salt was converted to its mesylatederivative by reaction with MsCl and Et3N in CH2Cl2, and themesylate intermediate was cyclized in hot CHCl3 in the presenceof excess Et3N to deliver 85 in 55% overall yield. Finally,hydrogenation of hexacyclic diguanidine85 over Rh‚Al2O3 inacidic MeOH provided batzelladine F isomers37and86, neitherof which was identical to the natural product by HPLCcomparison.

The stereoisomers of the proposed structure of batzelladineF that remained to be synthesized were36 and38, which wereepimeric at C16 to the isomers prepared thusfar (Figure 2). Thesynthesis of these compounds, which paralleled that of37, isdetailed in the Supporting Information. To our chagrin, theseisomers were also distinct from an authentic sample of batzel-ladine F by HPLC comparisons.

V. What is Batzelladine F?We had made one enantiomerof what we believed to be the four “possible” structures ofbatzelladine F that differed in relative configuration, howevernone were identical by HPLC comparisons to the naturalproduct. Attention next turned to the remote possibility that theguanidinium counterions were complicating these comparisons.Batzelladine F was isolated as its diformate salt, and retainsthese counterions strongly. Thus, we converted synthetic ster-eoisomer37, a ditrifluoroacetate salt, to its diformate salt bywashing a CHCl3 solution of 37 with aqueous formic acid-sodium formate solution (0.5 M each). Complete conversion tothe formate salt was confirmed by19F NMR analysis byobserving disappearance of the signal at-76.8 ppm. Asexpected this diformate salt had the same HPLC retention timeas the corresponding ditrifluoroacetate salt.

Having confirmed that the counterion would be unlikely tointerfere with HPLC comparisons, our focus turned to a care-ful examination of the small sample of authentic batzella-dine F that we had obtained from the SKB sample collection.Although some impurities were apparent, it was clear from its

1H NMR spectrum that this sample had not decomposedappreciably. In particular, the major component retained theoriginal axial orientation of the ester as evidenced by thechemical shift and coupling constants of the methine hydrogenadjacent to the ester:δ 3.06 (dd,J ) 4.6, 3.3 Hz, CD3OD).As expected, the major peak in the HPLC chromatogram ofthe authentic sample possessed the expected exact mass byelectrospray ionization (m/z 625.5161; 625.5169 calcd forC37H65N6O2). However, the mass spectrometry (MS) fragmenta-tion pattern of authentic batzelladine F differed from those ofthe isomers we had prepared. Our samples showed peaks atm/z ) 304 and 322 resulting from the expected McLaffertyfragmentation of the ester bond, whereas the synthetic samplelacked these fragments and showed peaks atm/z ) 276 and350.43

Now suspicious of the constitution of natural batezelladineF, we subjected both synthetic isomer37 and authenticbatzelladine F to basic methanolysis followed by MS analysis.Synthetic37 yielded the expected peaks atm/z ) 294 and 364,corresponding to alcohol81 and ester87 (Scheme 20). Yet theauthentic sample yielded peaks atm/z ) 322 and 336 (Scheme21). As one product was 28 amu heavier than expected, whereasthe other was 28 amu lighter, it became apparent that the lengthsof the guanidine side chains in the originally proposed structure

(43) For a discussion of the McLafferty rearrangement in other batzelladinealkaloids see (a) Patil, A. D.; Kumar, N. V.; Kokke, W. C.; Bean, M. F.;Freyer, A. J.; De Brosse, C.; Mai, S.; Truneh, A.; Faulkner, D. J.; Carte,B.; Breen, A. L.; Hertzberg, R. P.; Johnson, R. K.; Westley, J. W.; Potts,B. C. M. J. Org. Chem. 1995, 60, 1182-1188. (b) Braekman, J. C.; Daloze,D.; Tavares, R.; Hajdu, E.; Van Soest, R. W. M.J. Nat. Prod. 2000, 63,193-196.

Scheme 18 Scheme 19



were incorrect. It seemed likely that the methanolysis productsof batzelladine F are88and89. Union of these fragments yieldsstructure90 (Figure 2), which fits all of the NMR and massspectrometric data for batzelladine F.

Conclusion

Stereoselective synthesis of octahydro-5,6,6a-triazaacenaph-thalenes having the anti relationship of the angular hydrogensflanking the pyrrolidine nitrogen confirmed suspicions8 thatthe relative configuration of the left-hand tricyclic guani-dine fragment of batzelladine F had been assigned incorrectly;1

this unit should be revised to have the syn relationship ofthese hydrogens. A convergent synthesis strategy was de-vised, whose central step was a fragment-coupling tethered-Biginelli reaction, that allowed one enantiomer of the eighthexacyclic diguanidines depicted in Figure 2 to be prepared.These synthetic products differed from batzelladine F, there-fore the mass spectrum of natural batzelladine F was rein-vestigated. This analysis established that the originally pro-posed connectivity of this alkaloid also was incorrect, leadingus to propose the revised costitution90 of natural batzelladineF.

As the absolute configuration of the two tricyclic guanidinefragments of batzelladine F was still unknown and the relativeconfiguration at C18 (Figure 2), eight possibilities (four enan-tiomer pairs) remained for the structure of batzelladine F. Thetotal synthesis of one enantiomer of these four diastereomersand the comparisons of these products with the natural marineisolate that firmly established90as the correct constitution andrevealed the full three-dimensional structure of batzelladine Fare described in the accompanying article.9b

Acknowledgment. We thank NIH (HL-25854) for financialsupport, Pharmacia and the ACS Organic Division for fellowshipsupport (F. C.), Dr. John Greaves for mass spectrometricanalyses, and Dr. J. A. Chan of SmithKline Beecham forproviding an authentic sample of batzelladine F, without whichthe structural revision would not have been possible. NMR andmass spectra were determined at UC Irvine with instrumentspurchased with the assistance of NSF and NIH.

Supporting Information Available: Reaction schemes for thepreparation of compounds reported only in Supporting Informa-tion, experimental details and tabulated characterization datafor new compounds, HPLC chromatograms of repuified naturalbatzelladine F and synthetic batzelladine analogues36, 37, 38,86, S14, andS16, ESI mass spectrometric analysis of syntheticbatzelladine analogue37 and authentic batzelladine F, copiesof 1H and13C NMR spectra of new compounds, and completeref 16 (125 pages, print/PDF). This material is available free ofcharge via the Internet at http://pubs.acs.org.

JA0574320

Scheme 20 Scheme 21



evolution of a strategy for the synthesis of structurally complex batzelladine alkaloids....

Documents